In the prior art, with an injection molding machine, dies (a fixed die and a movable die) are attached to a fixed platen and to a movable platen, and the movable platen is driven in directions to approach towards the fixed platen and to be withdrawn therefrom, and thereby closing and opening of the dies are performed.
Synthetic resin in the molten state is supplied to a cavity within the dies that are closed together, and this synthetic resin in the molten state hardens so that a molded product is formed; and, after opening of the dies, this molded product is ejected from the mobable die by an ejector device.
In an ejector device of an injection molding machine, an ejector plate is disposed at the rear side of the movable platen, a plurality of ejector pins (i.e. ejector rods) are inserted through the movable platen so as to slide freely therein, and the base end portions of these ejector pins are fixed to the ejector plate. By the ejector plate being driven in the directions to approach the platen and to be withdrawn therefrom, the ejector pins are driven forwards and backwards between eject positions in which they are advanced and wait positions in which they are retracted.
A construction is known (for example, refer to Patent Document #1) in which the ejector pins are made as single continuous unitary type pins, with the base end portions of these ejector pins being fixed to the ejector plate and with the ejector pins piercing through the ejector plate and a retainer plate being engaged to the ejector plate from its rear side by a plurality of bolts, so that base end flange portions of the ejector pins are sandwiched between the ejector plate and the retainer plate; and also a construction is known (for example, refer to Patent Document #2) in which the base end portions of the ejector pins are engaged to the ejector plate by being screwed thereinto.
And, with regard to the ejector device of an injection molding machine described in Patent Document #3, an air cylinder is fixed to the ejector plate so as to face towards the platen, and base end flange portions of the ejector pin is linked via joint to the cylinder rod of the air cylinder. The base end flange portion of the ejector pin is engaged to concave portions of the joint in a manner so that their engagement can be released from directions orthogonal to the axis, and is fixed by a bolt so as not to rotate. The ejector pin can be exchanged by removing the bolt.
With regard to the ejector device of an injection molding machine described in Patent Document #4, each ejector pin includes tip end side ejector pin passing through a incorporated die that is assembled to the die, and a base end side ejector pin whose base end portion is fixed to the ejector plate, and the tip end side ejector pin is pressed and driven by the base end side ejector pin. In order to prevent operational faults due to thermal expansion of the embedded portion, the construction is adapted to permit some shifting of the tip end side ejector pin in directions orthogonal to the axis.
Patent Document #1: Japanese Laid-Open Patent Publication No. 5-245847; Patent Document #2: Japanese Laid-Open Patent Publication No. 2000-301580; Patent Document #3: Japanese Laid-Open Patent Publication No. 2007-98957; Patent Document #4: Japanese Laid-Open Patent Publication No. 2005-254514.
In regard to a conventional ejector device of the injection molding machine, when exchanging the die on the injection molding machine, sometimes it becomes necessary to exchange the ejector pins, in order to change the disposition the ejector pins to correspond to the new die (i.e. to the shape and size of new die). However, with a construction in which each ejector pin is made as a single continuous unitary type pin whose base end portions of the ejector pint is fixed to the ejector plate as in Patent Documents #1 and #2, the task of exchanging the ejector pins is heavy, the time and cost required for exchanging the ejector pins are also great and expensive.
In other words, regarding the ejector device described in Patent Document #1, the retainer plate must be removed from and fitted back to the ejector plate when exchanging the ejector pins, and due to this a plurality of bolts must be operated and the ejector pins must be removed and fitted from the rear side of the retainer plate, and this requires a lot of work and trouble.
Moreover, regarding the ejector device described in Patent Document #2, when exchanging the ejector pins, the ejector pins are rotated and the base end portions of the ejector pins are disengaged from and engaged to the ejector plate, and in particular, when engaging the base end portions of the ejector pins to the ejector plate, it is necessary to perform positioning of the base end screw portions of the ejector pins against the screw holes in the ejector plate and to exert a large force for rotating the ejector pins in this state, which imposes a great burden. Moreover there is a possibility that, at this time, the ejector plate may be damaged.
And, with regards to the ejector device described in Patent Document #3, the base end flange portion of the ejector pin is removably engaged to the concave portion of the joint that is linked to the air cylinder from directions orthogonal to the axis, and is fixed by a bolt being screwed tight therein, and it is possible to remove the bolt in order to exchange the ejector pin, but it is difficult to shift the base end flange portions of the ejector pin in the directions orthogonal to the axis in the state in which the ejector pin is passed through the platen, in other words to perform the linking to the joint and the release of such linking; and it is also quite absurd to provide a plurality of air cylinders that include air cylinders that are not used, in order to correspond to changes of configuration of the ejector pins.
And, in the case of a medium or large sized injection molding machine (some hundreds of tons class or one thousand and some hundreds of tons class), it often happens that the arrangement of the ejector pins must be changed when exchanging the die, so that the task of exchanging the ejector pins is required repeatedly. However, the lengths and the thicknesses of the ejector pins are great, and for example, with an injection molding machine of approximately 650 tons, the diameters of the ejection pins may be about 35 to 50 mm and their lengths may be about 1 m, so that the weight of the ejector pin also becomes great. Consequently, the burden of the task of exchanging the ejector pins becomes extremely great.
An object of the present invention is to provide an ejector device of a molding machine, which is capable of fixing a portion or whole of each ejector rod to the ejector plate by magnetism of one or more permanent magnets in a detachable manner, thereby makes it possible to reduce the burden and cost of the task of ejector rod exchange.